The optimization of the light outcoupling from an OLED is an important factor in the commercialization of OLEDs in many applications.
OLEDs have reached remarkable levels of power efficiencies over the years by lowering the operating voltage and increasing the quantum yield of internal light generation to almost 100% resulting in power efficiencies of more than 120 Lm/Watt for green light (Ultra High Efficiency Green Organic Light-Emitting Devices, Jpn. J. Appl. Phys. 46, p. L10-L12, (2007))|and recently 100 Lm/W for white light (Press Release of Jun. 17, 2008 by Universal Display Corp.).
In this application, we will discuss OLED displays and OLED light sources which are made of pixels or stripes which can emit the primary colors R, G, and B. For a display, by varying the intensity of each of these components, all of the colors of the visible spectrum can be generated. In the case of white light sources, the intensity of the R, G and B pixels can be varied to get the optimal white light to address the lighting requirements for a particular lighting application. By increasing the red component, the white light can be made ‘warmer,’ and by increasing the blue component, the white light will appear to be ‘colder’.
The high efficiency is a major advantage not only for the use of OLEDs for displays, but also their use as a light source for signage and general lighting purposes.
Nevertheless, the efficiency could be much higher if all the light which is generated within the OLED device were actually being coupled out of the device. Currently, this efficiency is only about 20-25%, due to the planar waveguiding properties of the OLED device.
Efforts have been made to enhance this light out-coupling by putting optical stacks and/or scattering layers on top of the device. However, these methods do not necessarily result in optimized light output.
A multilayer mirror formed by a quarter-wave stack only enhances one narrow band of wavelengths of light emitted from the OLED. The structure in a quarter wave stack is wavelength dependent, which means that the use of the same structure for RGB applications can be problematic. Also, the number of layers needed for an effective quarter wave (QW) stack can be quite high (e.g., 10 layers pairs), which makes it impractical for many commercial applications. Furthermore, the OLED emission is generally broad. In a well designed QW stack, it is possible that a portion of the light will not be transmitted. Another disadvantage of the structure is that the intensity of the light transmitted by a QW stack is strongly directional, and in general the stack is designed with maximum intensity for forward transmission. This feature is generally unfavorable for display applications because it limits the viewing angle and is unacceptable for general applications in solid state lighting (SSL). The implementation of effective QW stacks requires strict control of thickness uniformity (Δt<5 nm). This is generally achieved with a thin layer (t<20 nm) deposited at relatively low speed. It would become impractical for thick layers (100 nm<t<1000 nm), as for example, the layers required to form an efficient environmental barrier.
Optical cavities (single and multiple) have been suggested to enhance the light output of OLED devices. Similarly to QW stacks, optical cavities are wavelength specific. Therefore, a specific design and its accurate implementation are necessary for each wavelength. The emission from a single optical cavity is very angular dependent. Double or triple cavities must be implemented to make the emission independent from the observation point. The implementation is therefore practically difficult and complex and may require a different sequence of layers as well as different thicknesses of the layers in parts of the device.
Another problem with using OLEDs in flat panel displays and other information display formats is that they are limited by the poor environmental stability of the devices. G. Gustafson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature, Vol. 35, 11 Jun. 1992, pages 477-479. Humidity and oxygen significantly reduce the useful life of most OLEDs. As a result, these devices are typically fabricated on glass substrates with glass covers laminated on top of the OLED and with the edges sealed to exclude water and oxygen from the active layers.
Thus, there remains a need for a method of improving the light outcoupling of encapsulated OLEDs compared to the bare OLED, while protecting the OLED from environmental contaminants.